Thanks brett.
This part would be ok just for the initial setup of the headerOffset.

Unfortunately it does not helps for changing it dynamically

$('#example').DataTable({
fixedHeader: {
header: true,
headerOffset: $('#fixed').height()
}
});
// some event which should result in new position of tableHeader
// due to change in height of page header
.... (function (e) {
var table = $('#example').DataTable();
// DO SOMETHING HERE TO UPDATE fixedHeader.headerOffset
table.fixedHeader.headerOffset($('#fixed').height()); // apply new value
// above fails, as there so no such method to update headerOffset (yet)
table.fixedHeader.adjust();
});